How to retrieve User ID and Password using Ember Js

My requirement: Consider a client server web application. We have an LDAP server to authenticate.

When user logins to the application from a browser, we need to retrieve the current windows system user ID and password and then pass it to LDAP server for authentication.

Question: Where in windows does the windows system username and password is stored? How can we retrieve the user ID and the password using Ember. Please note that its not the web application userID and password, its the windows system userID and password.